Tunable optical fiber bragg and long period grating

ABSTRACT

Disclosed is a tunable optical filter which may be used in telecommunications systems. The filter comprises a single mode optical waveguide containing a grating. The resonance wavelength of the filter is changed by changing the boundary condition at the interface between the cladding layer and an additional layer applied to the outer cladding layer surface. This boundary condition is changed by changing the refractive index of the additional layer. Means for changing the refractive index of the additional layer include establishing a structural resonance in the additional layer, or forming the additional layer from electro-optic or piezoelectric materials.

This application is based upon the provisional application Ser. No.60/079,873, filed Mar. 30, 1998, which we claim as the priority date ofthis application.

BACKGROUND OF THE INVENTION

The invention is directed to an optical filter which is tunable, i.e.adjustable, with regard to the center wavelength characterizing thefilters spectral response. In particular, the subject filter ispreferably tuned by either optical or electrical means.

Optical filters have become essential components in wavelengthmultiplexed communications systems and in systems which use opticalamplifiers. Such filters are used as elements to add or drop a selectedwavelength and so may be used to block broadband noise while passing asignal, to flatten optical amplifier gain, or to direct a signal ofparticular wavelength into pre-selected nodes. Greater systemflexibility can be achieved using filters whose wavelength response istunable over a range of wavelengths.

A number of alternative filtering devices are known including,Fabrey-Perot or Mach-Zehnder interferometers, multiple layer dielectricfilm filters, and filters based on waveguide Bragg or long periodgratings. Tuning of these devices may be accomplished by means thatchange the device refractive index or dimensions. For example straininga device, by bending or stretching a device or a portion thereof, canserve to alter both dimensions and refractive index. In a similar way,dimensions or index may be altered by altering the temperature of adevice or a portion thereof. Thermo-electric cooling and heating is aconvenient way to carry out thermal adjustment of a device. In addition,optical or electrical means can be used to alter device dimensions orrefractive index and thus the device filtering characteristics. Theselatter means are usually preferred because they provide a filter havinga faster response, and which is more reliable, and afford morereproducible control of the device, as compared to devices tuned bymechanical or thermal means.

There is therefore a need in the art for a tunable filter device having:

rapid response to a tuning means;

a high degree of reliability; and,

a high degree of reproducibility.

DEFINITIONS

An optical fiber grating is a periodically or quasi-periodicallyperturbed waveguide for electromagnetic radiation, the grating, i.e.,the perturbation having a preselected length along which the refractiveindex or the profile of the waveguide changes periodically.

The period of a grating is the distance between corresponding points intwo nearest neighbor high or low refractive index portions of thegrating.

A long period grating is one that provides a resonance between claddingmodes and a core mode propagating in the same direction.

A structural resonance occurs when electromagnetic waves, such as light,bounce around within a structure because of total or near-total internalreflection from a boundary between a high and a low index region andcomes back on itself in phase after a single or multiple reflections.Fabrey-Perot interferometers are the simplest example of one dimensionalstructural resonance. For structural resonance to occur in a waveguidein the transverse plane, the waveguide must be surrounded by a medium oflower refractive index than that of the waveguide. In a circularwaveguide, such as an optical fiber, structural resonance occurs withinthe cladding region because of the total internal reflection at theclad-air or clad-jacket interface. In the case of optical fiber, lightincident substantially normal to the usual direction of propagation getstotally internally reflected by the clad-air or clad-jacket interfaceand at certain wavelengths after many such reflections comes back onitself in phase to constructively interfere and hence cause a structuralresonance. (A good reference for this is: S. C. Hill & R. E. Benner,“Morphology Dependent Resonances”, in P. W. Barber R. K. Chang eds.“Optical Effects Associated With Small Particles”, World Scientific (NewJersey. 1988)). FIG. 5 shows an example of structural resonance that canoccur in an additional layer surrounding and in contact with thecladding layer of an optical waveguide. In this example, a laser is usedto direct light into the layer in a direction substantiallyperpendicular to the layer surface, The structural resonance of theincident light which occurs in the layer changes the intensity dependentterm of the refractive Index of the layer and so changes the peakwavelength filtered by an associated grating.

A Bragg grating is one which produces a resonance between a core modeand a counter-propagating, reflected core mode.

Throughout this document the term waveguide is taken to mean single modewaveguide unless expressly stated otherwise.

SUMMARY OF THE INVENTION

The tunable filtering device of this application meets the need for highperformance tunable filters by providing an optically or electricallycontrolled long period or Bragg grating device.

A first aspect of the invention is a tunable optical filter whichincludes a single mode optical waveguide having a grating impressed uponat least a portion of the waveguide core. The tunability derives from anadditional layer applied to the outer surface of the waveguide cladlayer. This additional layer is made of a material whose refractiveindex may be changed by a control mechanism which acts upon theadditional layer. Changing the refractive index of this outermost layer,changes the boundary conditions of the electromagnetic fields propagatedin the waveguide. This change in boundary conditions will affect thepropagation constant of the cladding modes. Depending upon the distanceof the additional layer from the core-clad boundary, the change inrefractive index of the additional layer may also affect the propagationconstant of the core mode. For a typical single mode waveguide thisdistance is in the range of about 5 μm to 10 μm. The resonancewavelength of the grating depends directly upon the propagationconstants of the resonating modes; Thus, changing the propagationconstant effectively changes the resonance wavelength of the grating,effectively tuning the resonant peaks of the grating.

An embodiment of the tunable filter has an additional layer which iselectro-optic, for example LiNbO₃. The refractive index of the layer canthen be changed rapidly and reproducibly by means of a voltage appliedacross the layer. The applied voltage effectively changes thepropagation constant of the cladding mode and thus changes the resonantwavelength peaks of the grating. This is one embodiment of the longperiod grating.

In a preferred embodiment of this aspect of the invention, a structuralresonance is established in the additional layer by directing light fromone or more light sources onto the layer, the direction of travel of theincident light being transverse to the long dimension of the layer. Atstructural resonance, light intensity becomes more concentrated in thelayer. The light intensity changes the intensity dependent term of therefractive index of the layer and so changes the propagation constant ofa cladding mode. The intensity dependent term is commonly called thenon-linear refractive index term. One writes the refractive index asn=n₁+n₂l, in which n₁ is the linear index, l is light intensity and n₂is the nonlinear index coefficient. The grating is effectively tunedfrom one wavelength peak to another by controlling the incident lightintensity. A typical light source is one or more lasers which directlight into the additional layer in a direction transverse to the longdimension of the layer.

As the non-linearity coefficient n₂ of the material of the additionallayer increases, the structural resonance induced index change in theadditional layer is greater, so that the effect of the change in theadditional layer on the propagating modes, either cladding or core,becomes greater. A typical non-linearity coefficient of a dispersionshifted waveguide is about 2.3×10⁻²⁰ m²/W.

The effectiveness of the additional layer, as measured by the width ofthe tuning band, is expected to be enhanced in layer materials having arelatively higher nonlinear coefficient. The inventors contemplatecoefficients on the order of at least 10-19 at this time. Profilesdesigned to increase non-linear index coefficient are under study forexample in co-pending provisional application No. 60/071732 incorporatedherein by reference. A typical tuning band width is in the range ofabout 70 μm. Thus, a preferred embodiment of an additional layer, inwhich structural resonance is to be established, is an additional layercomprising a material having a non-linearity coefficient in the range ofabout 10⁻²⁰ 10⁻¹⁹ m²/W.

In yet another embodiment of the novel tunable filter, the additionallayer comprises a dye doped silica glass. The refractive index of such adye doped glass may be changed by launching light transversely into theglass, thereby tuning the wavelength of the filter.

To avoid interaction of the transversely launched light with the signallight propagating in the waveguide, the wavelength of light, used tochange the refractive index of the additional layer by means ofstructural resonance or interaction with a dye, is preferably outsidethe range of about 1300 nm to 1700 nm, which is an operating band ofoptical communication systems.

In yet another embodiment of the tunable filter, the additional layercomprises a piezoelectric material, for example the material may be asoft polymer. The density of the material, and thus the refractive indexof the material, can be changed by applying a voltage across thematerial, thereby tuning the grating to a different resonancewavelength.

In an embodiment of this first aspect of the novel tunable filter, Inwhich the boundary of additional layer is sufficiently close to the modepropagating in the core to change the propagation constant thereof, asis noted above, the grating period may be chosen to be that of a Bragggrating,

In a second aspect of the invention, the waveguide, having a core and aclad and an additional outermost layer, contains a grating of periodΛ_(g) which is chosen such that the difference in the propagationconstant of a cladding mode, β_(cl), and the propagation constant of acore mode, β_(c), are related by the equation β_(cl)−β_(c)=2π/Λ_(g), thecondition which defines resonance between the modes. Then the filter maybe tuned by changing β_(cl). The β_(cl) may be changed by changing therefractive index of the additional layer by any of the means notedabove.

In a third aspect of the novel tunable filter, the grating constant maybe chosen as Λ_(b), a constant appropriate for Bragg grating. Theresonance which is established is then governed by the equation,β_(r)−β_(c)=2π/Λ_(b), where β_(r) is a reflected mode. In this aspect,the filter is tuned by changing β_(c). Thus the thickness of thecladding layer much be chosen small enough to allow interaction betweenthe core mode and the additional layer-cladding layer boundary. Thenβ_(c) may be changed by changing the refractive index of the additionallayer by any of the means noted in the first or second aspect of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart of a data set which demonstrates the effect on a longperiod grating resonance of a change of the refractive index of theadditional layer.

FIG. 2 is a cross sectional view of an optical waveguide fiber havingthe additional layer.

FIG. 2a is a cross sectional view of an alternative waveguide shape.

FIG. 3 is a cross sectional view of a waveguide fiber having theadditional layer and an voltage applied to a length of that layer thatlayer.

FIG. 4 is a cross sectional view containing the long axis of a waveguidefiber having the additional layer together with means for exciting astructural resonance.

FIG. 5 is a cross sectional view of the waveguide containing thegrating. The effect of structural resonance on the nonlinear index ofrefraction is illustrated.

DETAILED DESCRIPTION OF THE INVENTION

High bit rate telecommunication links, for example those which make useof wavelength division multiplexing or optical amplifiers, requireeffective means for filtering a wavelength or a band of wavelengths.Further, in many applications, for example those which involve two waycommunication or multi-channel delivery to a node, the wavelength whichis to be passed or filtered changes with time. Thus the need exists fora filter whose characteristic wavelength may be changed, i.e., tunedover a reasonably broad band of wavelengths. The tuning bandwidth of thepresent invention is in the range of about 70 μm.

In the present invention, the filtering properties of waveguide gratingsare used together with means to change the resonance wavelength of thegrating to provide just such a tunable wavelength filter. The resonancewavelength may be changed by changing the dimensions of the grating, forexample the spacing of regions of high and low refractive index. As analternative, the resonance wavelength may be changed by changing therefractive index of the base glass containing the grating. Hence awaveguide grating may be tuned by applying stress to the waveguide andso changing its refractive index. Also, the change of refractive indexwith temperature may be used to alter the resonance condition. However,in cases in which response time is important or a high degree ofreliability and reproducibility are required, an alternative tomechanical or thermal tuning of the grating is needed. The novel tunablegrating of this invention alters the grating resonance by changing thepropagation constant of pre-selected modes in the waveguide. Thepropagation constant of a waveguide is found by solving the waveequation for the potential using boundary conditions appropriate to theparticular geometry and materials of the waveguide. For example, in thecase of a waveguide fiber having a core, a clad and a coating, the waveequation may be written in cylindrical coordinates and solved such thatthe solution field and its first derivative satisfy the usual conditionsat the interfaces of core and cladding and cladding and coating. Thecoating may be another glass or a polymer. Also, additional layers ofglass or polymer may be added to protect the waveguide from mechanicaldamage or impart to the waveguide additional desired properties.

The grating may be configured as a Bragg grating in which case theresonance occurs between a forward propagating wave and a wave that hasbeen reflected by the grating. As an alternative, the grating may have along period spacing in which case the resonance occurs between a forwardwave propagating in the waveguide core and a forward wave propagating inthe waveguide cladding. In terms common in the art, the resonance occursbetween a core mode and a cladding mode which are propagating in thesame direction. The condition for resonance in this latter case is thatthe difference in propagation constant of the core and cladding modeequal 2π times the inverse of the grating period. That is,β_(cl)−β_(c)=2π/Λ_(g), where the β's are the respective cladding andcore more propagation constants and Λ_(g) is the grating period.

Given this condition, the grating may be tuned by changing one or bothof the propagation constants. The Bragg grating, which has an analogousresonance condition may be tuned by changing the core mode propagationconstant. The problem of providing a tunable filter which has a fastresponse time and which is reliable and reproducible has been reduced tothe problem of finding fast and reliable means for changing thepropagation constant of the core or cladding modes in the waveguide.Changing the refractive index of the outer layer serves to change thesolution to the wave equation which describes the fields in thewaveguide and so change the propagation constant associated with modespropagating in a particular region of the waveguide, i.e., the core orthe cladding or both.

In FIG. 1 is illustrated the effect of changing the index at the outersurface of the clad layer. The solid curve 2 is a chart of thetransmission characteristics of the waveguide versus wavelength. Thedashed curve 4 shows the filtering effect produced by forming a longperiod grating in the waveguide, Curve 4 was measured in the case inwhich the outer clad surface was bounded by air. The waveguidecontaining the grating was then immersed in water and the transmissioncurve 6 was measured, Note that the change in index at the outer cladsurface shifted the filtered wavelength downward by about 2 nm ascompared to the filtered wavelength of curve 4.

Because the distance between the clad surface and the core to cladinterface was large relative to the signal wavelength in this case, thepropagation constant of the core mode was substantially unaffected bythe change in outer Clad surface boundary condition. The change in indexserved to change the propagation constant of one or more cladding modesand so change the resonant wavelength of the grating. The shift can beseen more clearly in inset 8 of FIG. 1. Because there is no apparentshift for the lower wavelength resonances 10, it is probable that theindex change affected a single cladding mode.

The cross sectional view in FIG. 2 of a waveguide fiber, having agrating, in accordance with the present invention, shows the core region12 surrounded by the cladding layer 14. An additional layer 16 is formedabout the outer surface of cladding layer 14. In this configuration, achange in the refractive index of additional layer 16, changes theboundary conditions of the propagated fields and so changes thepropagation constant and the resonant wavelengths of the grating.Depending upon the amount of refractive index change in the additionallayer 16 and the spacing between layer 16 and core 12, the changing ofthe refractive index in additional layer 16 can change the propagationconstant of core as well as cladding modes. It will be understood thatthe cross sectional drawings are not to scale.

An alternative embodiment of the tunable filter is shown in FIG. 2a. Inthis embodiment the rectangular shaped cores 19 are embedded in claddinglayer 20. The propagation constant of modes in cladding layer 20 ischanged in refractive index of additional layers 22. As before, theinfluence the additional layer has upon cladding modes depends upon theamount of refractive index change. The influence of the additional layeron the core modes depends upon the amount of the index change and thespacing between cores 19 and additional layer 22.

To increase the effect of the additional layer on the core modes, thecladding layer thickness can be reduced, either during manufacture ofthe waveguide cladding or by etching or grinding or otherwise reducingthe cladding layer thickness after the cladding layer has been formed.In order for the additional layer to affect the core modes, the spacingbetween core-cladding interface and cladding-additional layer interfaceis in the range of about 5 μm to 10 μm.

Additional layers that change density or otherwise change refractiveindex under the influence of an electric field are contemplated. In FIG.3 is shown additional layer 16 having a voltage applied by electricalcircuit 24 along a portion of its length. Given the proper choice ofmaterial, layer 16 will change refractive index as the applied voltageis changed. The change in refractive index in turn changes the boundarycondition at the cladding outer surface and so changes the propagationconstant of one or more cladding or core modes, thereby providing atunable grating. A different view of the applied voltage across thelonger dimension of the additional layer 16 is shown as circuit 26 inFIG. 4.

Another embodiment, one in which a structural resonance is establishedin layer additional 16, is illustrated in FIG. 4. In this embodiment,the difference in refractive index between cladding layer 14 andadditional layer 16 is large enough to produce total internal reflectionof light incident on the outer surface of the additional layer andrefracted into the additional layer. As previously discussed, thestructural resonance concentrates light intensity in the additionallayer 16 and changes the nonlinear refractive index of the additionallayer, thereby providing a tunable grating, indicated schematically bythe lines 28. Note that individual segments of the grating need notalternate symmetrically. Apodization techniques, which involvesuperimposing a broad index envelope along the grating length, may beused. Light sources 30 in FIG. 4 may pass through an optical element 32which may serve, for example, to distribute light intensity evenly alongthe additional layer. Sources 30 could be lasers for example. Theoptical element between the light source and the additional layer may bea lens as illustrated by object 34 in FIG. 4.

The relative indexes of the materials abutting the layer 16, whichresult in structural resonance, are further illustrated in FIG. 5. Cladlayer 14 abuts the inside surface of layer 16. The clad layer may have arefractive index equal to, less than, or greater than that of layer 16as shown in respective index diagrams, i.e., charts of refractive indexversus radius, 36, 38 and 39 of FIG. 5. The refractive index of thematerial or vacuum abutting the outside surface of layer 16 must have anindex lower than that of layer 16. The raised index of the core region12 is shown as curves 40 in the index diagrams. The light intensitydependent portions of the index are shown as curves 42. In the caseshown, the clad layer index is illustrated as horizontal portions 44.

It will be understood that the invention includes combinations of meansfor changing the refractive index of the additional layer. For example,additional layer 16 could comprise a piezoelectric polymer. A voltagecould be impressed across the polymer layer, thereby changing itsrefractive index, and a structural resonance using a light source couldalso be employed.

Although particular embodiments of the invention are hereinabovedisclosed and described, the invention is nonetheless limited only bythe following claims.

We claim:
 1. A tunable optical filter comprising: a single mode opticalwaveguide comprising a core region having a length and a cladding layercontiguous to the core region, the core region length having a sequenceof adjacent sublengths which make up at least a portion of the corelength, the sequence of sublengths having alternating higher and lowerrefractive index to form a grating, the grating having an optionalapodization envelope; an additional layer contiguous to the claddinglayer and extending along the cladding layer for at least a portion ofthe sequence of sublengths which form the grating, the additional layercomprising a material having a refractive index and a non-linearitycoefficient in the range of about 10⁻²⁰ to 10⁻¹⁹ m²/W; and, means forchanging the refractive index of the additional layer.
 2. The tunableoptical filter of claim 1 in which the grating is a long period grating.3. The tunable optical filter of claim 2 in which the additional layeris an electro-optic material and the means for changing the refractiveindex of the additional layer is the application of a voltage across theadditional layer.
 4. The tunable optical filter of claim 3 in which theelectro-optic material is LiNbO₃.
 5. The tunable optical filter of claim2 in which the additional layer comprises a material having a lightintensity dependent refractive index, the outer surface of the layersurrounded by a material of refractive index lower than that of theouter layer to provide for structural resonance in the additional layer,the structural resonance being induced by directing a light source intothe additional layer in a direction transverse to the direction alongwhich the additional layer extends.
 6. The tunable optical filter ofclaim 2 in which the additional layer comprises a dye doped silica glassand the means for changing the refractive index of the additional layeris laser light launched into the additional layer in a directiontransverse to the direction along which the additional layer extends,the laser wavelength selected to interact with the dye.
 7. The tunableoptical filter of claim 6 in which the wavelength of the laser light isoutside the wavelength range of about 1300 nm to 1700 nm, which is anoperating band of optical communication systems.
 8. The turnable opticalfilter of claim 2 in which the additional layer is a polymer whichexhibits a piezo-electric effect and the means for changing therefractive index of the additional layer is a voltage applied across thepolymer to change the density thereof.
 9. The tunable optical filter ofclaim 1 in which the grating is a Bragg grating.
 10. The tunable opticalfilter of claim 9 in which the additional layer comprises a materialhaving a non-linearity coefficient in the range of about 10⁻²⁰ to 10⁻¹⁹m²/W.
 11. A tunable optical filter comprising: an optical waveguidecomprising a core region having a length and a cladding layer contiguousto the core, the core region having a sequence of adjacent sublengthswhich make up at least a portion of the core length, the sequence ofsublengths having alternating higher and lower refractive index to forma grating having a period A_(g), the grating having an optionalapodization envelope, in which, the core has an associated propagationconstant and the clad layer has an associated propagation constantB_(cl) and B_(cl)−B_(c)=2π/A_(g); and, means for changing B_(cl). 12.The tunable optical filter of claim 11 in which the means for changingB_(cl) comprises, an additional layer contiguous to the cladding layerand extending along the cladding layer for at least a potion of thesequence of sublengths which form the grating, the additional layercomprising a material having a refractive index; and, means for changingthe refractive index of the additional layer.
 13. The tunable opticalfilter of claim 12 in which the additional layer is an electro-opticmaterial and the means for changing the refractive index of theadditional layer is the application of a voltage across the additionallayer.
 14. The tunable optical filter of claim 13 in which theelectro-optic material is LiNbO₃.
 15. The tunable optical filter ofclaim 12 in which the additional layer comprises a material having alight intensity dependent refractive index, the outer surface of thelayer surrounded by a material of refractive index lower than that ofthe outer layer to provide for structural resonance in the additionallayer, the structural resonance being induced by directing a lightsource into the additional layer in a direction transverse to thedirection along which the additional layer extends.
 16. The tunableoptical filter of claim 15 in which the additional layer comprises amaterial having a non-linearity coefficient in the range of about 10⁻²⁰to 10⁻¹⁹ m²/W.
 17. The tunable optical filter of claim 12 in which theadditional layer comprises a dye doped silica glass and the means forchanging the refractive index of the additional layer is laser lightlaunched into the additional layer in a direction transverse to theextend of the additional layer, the laser wavelength selected tointeract with the dye.
 18. The tunable optical filter of claim 17 inwhich the wavelength of the laser light is outside the wavelength rangeof about 1300 nm to 1700 nm, which is an operating band of the opticalcommunication systems.
 19. The tunable optical filter of claim 12 inwhich the additional layer is a polymer which exhibits a piezo-electriceffect and the means for changing the refractive index of the additionallayer is a voltage applied across the polymer to change the densitythereof.
 20. A tunable optical filter comprising: an optical waveguidecomprising a core region having a length and a cladding layer contiguousto the core to forma core-clad interface, the core region having asequence of adjacent sublengths which make up at least a portion of thecore length, the sequence of sublengths having alternating higher andlower refractive index to form a grating having a period A_(b), thegrating having an optional apodization envelope, in which, the core hasan associated propagation constant B_(c) for light propagating in afirst direction in the waveguide and an associated propagation constantB_(r) and B_(r)−B_(c)=2π/A_(b) and, means for changing B_(c).
 21. Thetunable filter of claim 20 in which the means for changing B_(c),comprises, an additional layer contiguous to the cladding layer forminga clad-additional layer interface, which has a thickness, and extendingalong the cladding layer for at least a portion of the sequence ofsublengths which form the grating, the additional layer comprising amaterial having a refractive index; and, means for changing therefractive index of the additional layer, the cladding layer thicknessbeing pre-selected so that a change in refractive index of theadditional layer produces a change in B_(c).
 22. The tunable opticalfilter of claim 21 in which the additional layer is an electro-opticmaterial and the means for changing the refractive index of theadditional layer is the application of a voltage across the additionallayer.
 23. The tunable optical filter of claim 22 in which theelectro-optic material is LiNbO₃.
 24. The tunable optical filter ofclaim 21 in which the additional layer comprises a material having arefractive index sufficiently lower than the refractive index of thecladding layer to provide for structural resonance in the additionallayer, the structural resonance being induced by directing a laser beaminto the additional layer in a direction transverse to the direction inwhich the additional layer extends.
 25. The tunable optical filter ofclaim 21 in which the additional layer comprises a material having anon-linearity coefficient in the range of about 10⁻²⁰ to 10⁻¹⁹ m²/W. 26.The tunable optical filter of claim 21 in which the additional layercomprises a dye doped silica glass and the means for changing therefractive index of the additional layer is laser light launched intothe additional layer in a direction transverse to the direction in whichthe additional layer extends.
 27. The tunable optical filter of claim 26in which the wavelength of the laser light is outside the wavelengthrange of about 1300 nm to 1700 nm, which is an operating band of theoptical communication systems.
 28. The tunable optical filter of claim21 in which the additional layer is a polymer which exhibits apiezo-electric effect and the means for changing the refractive index ofthe additional layer is a voltage applied across the polymer to changethe density thereof.
 29. The tunable optical filter of claim 21 in whichthe spacing between the core-cladding interface and thecladding-additional layer interface is in the range of about 5 μm to 10μm.